1. Technical Field
The present invention relates to the measurement of the diameters of small holes formed in a substrate and more specifically relates to an improved system for the automatic measurement of the diameters of small holes formed in a plate such as the orifice diameter of a shower plate used for the disperse supply of a fluid (gas) in, for example, the vacuum chamber of a semiconductor manufacturing apparatus or the like. More concretely, the present invention relates to a small hole diameter measurement system for a plate in which the effective internal diameters of a plurality of small holes formed in a single plate can be automatically measured quickly and with high precision.
2. Description of the Related Art
In semiconductor manufacturing equipment and liquid crystal displays, due to an increase in the size of the materials that are treated, it is necessary to maintain in-plane uniformity. Accordingly, for example, in vacuum chambers used in semiconductor manufacture, a technique is generally practiced in which a plate having numerous small holes is disposed in the inside upper part of the chamber, and various treatment gases are supplied to the top of the material being treated (e.g., silicon wafer) via this plate.
A plate of this type is ordinarily formed from a stainless steel material, ceramic material or the like with a thickness of approximately 3 to 30 mm and an external diameter of approximately 100 mm to 300 mm. Approximately 20 to 100 small holes having a circular cross-sectional shape with an internal diameter of approximately 50 μm to 3000 μm are formed either regularly or irregularly in this plate.
The above-described small holes are formed by mechanical working or chemical working, and the working precision is restricted to a precision of ±1 μm (error dimension) or better.
Since the diameter of the small holes in the above-described plate is directly related to the amount of treatment gas that is caused to jet into the chamber, this diameter must be strictly controlled; and various techniques for this have been developed in the past.
Among these techniques, (a) a method in which compressed air is supplied from one end of the small holes, and the internal diameter of the small holes is detected from the variation in the air back pressure on the outlet side of the small holes (e.g., Japanese Patent Application Laid-Open (Kokai) No. 2003-65742), and (b) a method in which the diameter of the small holes is detected from the flow rate of the gas that flows through the small holes (e.g., Japanese National Patent Publication No. H4-502666), have attracted attention as techniques that can be adapted for practical use.
However, in the case of the method of (a) described above, the direct object of the method is the measurement of the internal diameter of a pipe-form body that has a relatively small diameter; accordingly, in cases where the object of measurement consists of small holes with a circular cross-sectional shape that are formed in a plate, since the length of the small holes is relatively short, the following problem arises: namely, the pressure on the outlet side of the small holes (back pressure) cannot be measured simply and with a high degree of precision.
Meanwhile, in the case of the latter method of (b), as shown in FIG. 8, an air current is caused to flow into the small holes that are the object of detection under critical conditions, the temperature of the air current is measured by a temperature detection device, the flow rate of the air is measured by a differential type flow meter, and the effective cross-sectional area of the small holes is calculated using the above-described measured value of the air flow rate and the above-described measured value of the air temperature.
In FIG. 8, the reference numeral 20 indicates a filter, 21 indicates a differential type flow rate measuring device in which numerous tube bodies 22 are combined side by side, 23 indicates the small holes that constitute the object of detection, 24 indicates a vacuum pump, 25 indicates a pressure gauge, and 26 indicates a thermometer.
In the measurement of the internal diameter of the small holes 23, first the air flow-through system is placed under so-called critical conditions by operating the vacuum pump 24 (P1/P2>2), the flow rate VE of the air that flows through the small holes 23 is measured by means of a differential type flow rate measuring device 21, and the air temperature T is measured.
The cross-sectional area A of the small holes is next calculated from the respective measured values using the following formula:A=K′·VE·[1−0.0017(T−20)]
wherein, K′ is a constant, VE is the measured value of the volumetric flow rate of the air, and T is the air temperature (° C.).
However, the method of (b) described above has a problem: a so-called differential type flow meter is used in which the volumetric flow rate VE of the air is calculated using the differential pressure ΔP of laminar flow regions that are formed by combining numerous tube bodies 22 side by side; accordingly, the construction of the flow meter itself becomes complicated, and it is difficult to measure the volumetric flow rate VE of the air with a high degree of precision.
Furthermore, in the method of (b) described above, the cross-sectional area A of the small holes 23 is calculated using a formula as described above. However, in cases where the length dimension of the small holes 23 is relatively long (e.g., 3 to 10 mm), and a plurality of areas where the cross-sectional area A varies are present in the longitudinal direction of the small holes, it becomes difficult to measure the cross-sectional area A of the small holes 23 accurately using the formula described above because of the effects of the viscosity of the air current and like, and it has been demonstrated that a large error is generated in verification tests conducted by the inventors of the present application.
Moreover, in the case of the method of (b), the replacement of the small holes 23 that constitute the object of inspection takes time and trouble; accordingly, if the object of inspection is numerous small holes formed in a plate, it is difficult to measure the internal diameter of the small holes efficiently and with a high degree of precision.
In addition, in the case of the method of (b), absolutely no consideration is given to the automatic replacement of the small holes 23 that constitute the object of inspection or the processing of the measured small hole diameter results (e.g., ascertaining the acceptability of the inspection results for the small hole diameters or transmission of the results of such evaluation); accordingly, such a method cannot easily be applied to actual semiconductor manufacturing plants.
Furthermore, it is difficult to control the flow rate of the gas that flows out of the small holes with a high degree of precision merely by controlling the internal diameter of the small holes. The reason for this is that the gas flow rate fluctuates greatly not only according to the internal diameter of the small holes, but also according to the degree of roundness, depth, presence or absence of a taper, and the like.